The FP-45 pistol was a small and specialized but potentially terrifying weapon. Technologically, it was simple: it fired just a single .45 caliber bullet down an unrifled barrel. It was so cheap that the United States could afford to manufacture hundreds of thousands of them and airdrop them over Nazi-occupied territory. The plan was that untrained civilians would use them to kill solitary German soldiers:

The pistols would be air dropped by the hundreds of thousands into enemy occupied territory, where it was expected the Germans would never be able to recover all of them. Useless as a battlefield weapon, the issue of providing useful weapons to the enemy was moot. On the other hand... as a weapon of terror in the hands of the resistance, the Liberator might have had extraordinary value. A common civilian, alone with a conquering German soldier, suddenly produces the single shot .45 and drops the man in a surprise attack, afterwards making off with the soldiers weapons. Now the German Army is down one soldier, the resistance has one more battle rifle, and every other German soldier has to wonder.... will he be next?

I say might have had, as the OSS never carried out the plan to any degree. Aside from a few FP-45's finding their way to the Philippine resistance and perhaps China, the Liberators were not deployed as expected. They languished in warehouses, and after the war... almost all were destroyed.

At the link, you can find more pictures and a video of a Liberator being fired.

I used to sell antique, rare, and class III guns. I have never seen one of these in person. If they're in good condition they can be worth a lot of money to a collector. Many of these pistols were destroyed in the impact of the drop. Also they were made from thin sheet steel, many surviving pistols rusted away.